Marine Biology

Temperature

Ocean is the largest store house of the sun’s heat and it occupies much space. This stored heat of the ocean is able to regulate the temperature of the world. The extremes of temperature range from – 3 to 40º C, while in the Indian seas the temperature ranges between 18 to 25º C at the surface. Seasonal variations of temperature in tropical waters are not much. There is always a direct stratification in sea and the temperature of the bottom water of the deep sea may be about – 1º C. The density of the sea water increases with decrease in temperature. Similarly the solubility of oxygen also increases generally with a lowering of temperature.

Seawater temperature affects marine organisms by changing the reaction rates within their cell(s). Although each species has a specific range of temperature at which it can live, the warmer the water gets the faster the reactions and the cooler the water the slower the reactions. An organism's response to water temperature is considered to be cold blooded (or poikilothermic) or warm blooded (homiothermic) depending on their ability to control their internal body temperature. If any species is moved out of its temperature tolerance range, it may die in a short time although temperatures on the cool side of the range are easier for organisms to tolerate than temperatures on the warm side because cell reactions just slow down in the cold but may speed up over six times the normal levels for each 10º C of heat.

Cold blooded (poikilothermic) marine organisms lack any body temperature regulatory controls. These include marine plants, invertebrates, most fish and marine reptiles. These species each have their specific temperature tolerance range within which they must live (some are adapted to polar temperatures, some to tropical temperatures). Some have a narrow range (and are thus very restricted) and some have a wide range (and are thus less restricted).

Warm blooded (homiothermic) marine organisms have some type of internal temperature controls to maintain a constant body temperature. These organisms include a few fish, all sea birds and mammals. This ability allows these warm blooded marine species to migrate over vast distances through various temperatures without problems and include some of the animals on Earth with the longest migrations.

Marine animals show a varied response to temperature. The stenothermal animals like reef corals, salps and heteropods are always found around 20º C. Eurythermal animals like Cardium and Arenicola are able to withstand wide ranges of temperature. Temperature difference in the sea though not very conspicuous yet acts as effective barrier for the distribution of animals.

Marine animals present certain structural variations in relation to temperature. The number of vertebrae in fishes of colder regions is more. The fish species of flounders have 35 vertebrae in the warmer regions while in the colder regions they may have up to 50. The cold water forms also show an increase in size. This is because it takes a longer time for the cold water forms to attain sexual maturity and thus they get a chance to grow till then. There are however a few exceptions to this rule. The sea urchin, Echinus esculentus, and the gastropod Urofolpinx cinerea, show a larger maximum size in warmer waters.

There is also an increase in respiratory rates in many marine organisms. In Mytilus edulis , the respiratory rate increases with temperature up to the optimum limit and then it slowly decreases. A similar behaviour is found in Calanus finmarchicus and Emerita sp.